Microchannel plates, referred to herein as "MCP", are used in imaging amplification applications. In a typical application, an optical image is directed onto a photocathode, which emits a relatively small number of electrons in areas where light strikes the photocathode. The emitted electrons constitute a "photoelectron image". The MCP amplifies the photoelectron image. In an image intensifier such as a so-called "night vision" device,the amplified electron image is converted to a photon image on a phosphor screen anode. In other applications referred to as image converters, the amplified electron image is encoded as charge pixels in a silicon anode target, typically a CCD or other silicon pixel devices. MCP's are also used in the fastest rise time and lowest time-jitter photomultiplier tubes, for charged particle and photon detection in a wide variety of physical science instrumentation, in streak cameras, as amplifiers for CRT beams in oscilloscopes. MCP's provide the virtues of high speed with sub-ns rise/fall times and transit time spreads less than 100 ps; high gain (typically 10.sup.3 -10.sup.6 /per stage); 2-D incident electron image preservation under amplification; immunity to magnetic fields, and compactness.
A typical Microchannel Plate (MCP) consists of an array of miniature channel electron multipliers 90 oriented parallel to each other, formed into a plate as illustrated in FIG. 1. Each channel electron multiplier is a tubular channel with an aspect ratio: EQU .alpha.=L/d&gt;20, (1)
where L is the length of the channel, typically equal to the thickness of the plate, and d is the diameter of the channel. The channels in glass-based MCP are typically 10-15 .mu.m in diameter, with a range of 5 .mu.m-100 .mu.m diameter, and .alpha. is typically between 40 and 100. The top and bottom of the plate are covered with metallic electrodes which create an electric field along the channel axis. Glass based MCP can be made from a lead containing glass. The lead glass matrix is formed by repeated drawing of a preform of etchable glass fibers clad with the lead glass until the fiber core shrinks to the desired diameter, whereupon the preform is sawed to the appropriate thickness and the fiber cores are etched away. The disc produced is heated in a reducing atmosphere to produce the secondary emitting surface or dynode layer and an underlying weakly conducting layer. Optionally, a thin layer of SiO.sub.2 or Si.sub.3 N.sub.4 may be deposited over this surface to provide a surface with an improved secondary electron yield and greater durability. FIG. 1 shows the overall schematic of the microchannel plate. In operation, a high voltage is applied between the front and back surfaces of the MCP. Typically, the resistance of the MCP is about 10.sup.9 .phi.. The electric field accelerates electrons in the channels and causes them to collide with the walls. The MCP operates by successive secondary emission of the electrons colliding with the walls of the channel, which have been activated to provide the dynode layer. At each collision, a secondary emission yield of electrons emitted per incident electron is approximated by: EQU .delta.=AV.sub.c.sup.1/2, (2)
where A is a proportionality constant and V.sub.c is the electron collision energy in eV. For a wide range of lead glasses used in MCP's, A.about.0.2 and .delta..sub.max .about.3.5 at 0.3 kV. The gain mechanism in a single channel is shown in FIG. 2. Primary electrons 92 impinge on the wall 94 of the channel, provoking emission of secondary electrons 96, which are accelerated along the channel and again impinge on the wall, provoking emission of further secondary electrons 98, and so on.
The gain of a channel is given by: EQU G=(AV/2.alpha.V.sub.o.sup.1/2).gamma. (3)
where EQU .gamma.=4.alpha..sup.2 (V.sub.o /V) (4)
and V is the total channel voltage, V.sub.o is the initial energy of the secondary electron, typically about 1 eV.
Gains for lead glass-based channels are normally about 10.sup.3 -10.sup.4 at V=1000 V depending on the activation processing. Setting d(lnG)/d.alpha.=0, we find extremes for the aspect ratio and gain as: EQU .alpha..sub.M =AV/(3.3V.sub.o.sup.1/2) (5)
and EQU lnG.sub.M =0.184A.sup.2 V
For typical MCP channels using activated Pb-glass, the gain G.sub.M is about 1500 at V=1 kV with the extreme aspect ratio .alpha..sub.M being about 60. When the aspect ratio increases beyond .alpha..sub.M (for example, the plate thickness increases while the hole diameter is constant), the gain saturates at the maximum gain value. When the gain is increased or sufficient numbers of electrons are incident, the space-charge at the output of the channel will prevent linear gain behavior. In typical glass-based pores, the space-charge limit is .about.10.sup.6 electrons for a 10 .mu.m pore. The space charge gain limit has been shown to be linear with the diameter (with V, .alpha. fixed).
A major problem for the MCP gain mechanism using reduced lead glass channels or other activation methods on glass is the decrease in gain with total charge drawn from the channel. The change in gain with use is an impediment to more widespread use of MCP, and is a major challenge for MCP manufacturers. Degradation of gain with operation is almost unavoidable with the chemistry of silica-based glasses, because of the evolution of and reaction with impurities in the channel by the electron bombardment of the channel surface.
It is also possible for the gain to increase before decreasing due to electron impact and heating during operation which cleans impurities and adsorbed gasses from the channel and increases the secondary emitter population. Such gain instability has limited the use of MCP in applications where stable gain is essential, and, in any case, limits the useful lifetime of MCP devices.
As set forth in U.S. Pat. No. 5,544,772, it has been proposed to make microchannel plate devices by light assisted electrochemical etching of n-type &lt;100&gt; silicon. The n-type silicon light assisted electrochemical etching process has distinct advantages over the conventional glass based manufacturing processes. However, there are significant disadvantages, which interfere with formation of deep (&gt;50 .mu.m) uniformly etched channels over large areas (such as 200 mm diameter silicon wafers). A further disadvantage of the n-type silicon electrochemical etching process is that a light source is required to generate holes so that etching may proceed.
As described, for example, in Lehmann et al, Formation Mechanism And Properties Of Electrochemically Etched Trenches In N-Type Silicon, J. Electrochemical Society, Vol. 137, #2, pp. 653-659 (1990) and in U.S. Pat. No. 4,874,484, light assisted electrochemical etching of n-type silicon produces deep channels perpendicular to the surface of the silicon. If the silicon surface is provided with pits at preselected locations, these channels form at the pits and hence at the same preselected locations. According to the Lehmann et al. article and in the '484 patent, the channels grow at their tips by selected etching of only the material at the tips of the channel. As described in these references and in numerous other references, it has long been believed that the mechanism responsible for such selective etching involves depletion of holes in the n-doped silicon and concentration of the few remaining holes at the tips of the channels due to the influences of electrical fields present in the process. Thus, the process of controlled deep channel growth at the preselected locations has been applied only in n-type silicon.
Propst et al., The Electrochemical Oxidation Of Silicon And Formation Of Porous Silicon In Acetonitrile, J. Electrochemical Society, Vol. 141, #4, pp 1006-1013 (1994) discloses the formation of deep channels at random locations using electrochemical etching of p-type silicon in a non-aqueous, anhydrous electrolyte. This reference does not disclose processes for forming deep channels at the pre-selected locations. Moreover, this reference emphasizes that aqueous electrolytes result in formation of highly branched, porous structures rather than deep, narrow channels of the types requires for microchannel plates and other applications. Similar teachings are found in Rieger et al., Microfabrication of silicon via Photoetching, the Electrochemical Society Proceedings, Vol. 94-361 (1994). Prost et al. U.S. Pat. 5,348,627 and 5,431,766 relate to the same work.
Despite these and other efforts in the art, there are substantial needs for further improvements in processes for forming deep, high-aspect ratio channels in silicon. It would be desirable to provide a process for forming channels in p-type silicon at preselected locations. P-type silicon wafers are fabricated in large numbers for use in manufacture of conventional silicon semiconductor devices. Therefore, p-type wafers are readily available at low cost. It would be desirable to be able to fabricate a device such as a microchannel plate having elongated channels therein at preselected locations starting from these economical, low cost wafers.
The processes used for forming channels in silicon heretofore have operated at the relatively low etch rates, so that the lengths of the channels increase at less than 1 .mu.m per minute. It would be desirable to form channels at a faster rate to reduce the the cost of the process.
Moreover, processes which require anhydrous electrolytes incur added costs due to the precautions which must be taken to eliminate water from the solvents and to isolate the process from moisture in the environment. These processes incure further costs associated with purchase and disposal of the required organic solvents. It would be desirable to eliminate these costs. Also, because hydrofluoric acid is difficult and expensive to handle, it would be desirable to provide a process which can be operated without the use of hydrofluoric acid as a starting reagent.
It would also be desirable to provide microchannel plates with electron-emissive dynode materials having enhanced resistance to degradation during use, so as to provide a microchannel plate with a more stable, long-lasting gain.